In evaporator system technology (and a wide range of liquid processing technologies), multi-pass tube and shell heat exchangers are a common method of heating liquids against saturated steam vapor from the evaporator process. The multi-pass fluid path on the tube side of the heat exchanger is optimized by design to provide turbulent liquid flow for best heat transfer and minimum pressure drop. Multiple flow passes are configured to provide adequate product velocity across the total heat transfer surface area required for the shell side heating medium (steam) to efficiently transfer heat into the product per the design case. The large open area tube shell is baffled to support the tubes and guide the steam vapor across the heat transfer surface towards a venting port. These relatively open tube shells are generally a multiple of the cross sectional area of the heat exchanger tubes and conducive to low pressure drop performance on the shell side.
As opposed to steam heating, there are often opportunities for liquid to liquid processing where a process stream of liquid medium is available for heating or cooling. A typical method for liquid to liquid heat transfer would be utilizing a plate heat exchanger (PHX), which comprises a large bundle of flat plates separated by narrow gaps that can be less than ⅛″ wide held apart by ridges that are pressed into the plates. PHXs are suited well for this type of process as the flow paths for product and heating medium are on opposite sides of the same heat transfer plate with equal cross sectional areas for the flow paths. Depending on flow rates available, PHXs can be passed in many variations to provide the best conditions for turbulence, required surface area, pressure drop, and heat transfer. The flexible design of the PHX is easily configured for true counter flow operation, all in a uniquely compact bundle of heat exchange surface. The down side to processing in a PHX is the inherent sensitivity to particulates and precipitates from the products processed in the PHX. PHXs are prone to plugging with unfiltered products, dense concentrates or products that precipitate crystals while being processed. PHXs are also not optimal for vacuum applications due to the leak potential of the numerous gaskets. Spiral heat exchangers are similar to plate heat exchangers in that they consist of a spiral wound pair of metal sheets separated by a gap with product and heating mediums on opposite sides of the same heat transfer plate, except with the plates configured into concentric spirals. Spiral heat exchangers generally have a wider gap between plates than plate heat exchangers, which improves their performance for products containing particulates and precipitates. However, spiral heat exchangers are more expensive to manufacture and still require a significant amount of gaskets. For this and other reasons, tube and shell heat exchangers are often preferred for the heat transfer duties that are required for evaporation systems and other applications. Tube and shell HX's are durable, vacuum tight, include a minimal number of gaskets (e.g., one at each end), can pass large particles and are much more tolerant of formation of fouling layers on the heat exchange surface.
The most common and effective liquid to liquid heat exchanger design being used today is the “Double tube” heat exchanger. These consist of a single pass product tube mounted inside of a slightly larger tube or “tube shell” (i.e., a tube inside a tube). Cross-sectional areas of the center tube and annulus of the tube shell are close to being equal in most cases providing a prime condition for single pass countercurrent heating or cooling of one liquid using another. Very simple in design and function but not suitable for large-scale flow duties, the racks of tubes required for large duties are cumbersome, space consuming, and expensive.
Historically, the standard construction of a multi tube or multi-pass tube & shell heat exchanger is not suited well for liquid to liquid heat transfer where it is beneficial to have comparable volumetric flow rates for both liquid streams. The total cross sectional area between the inner wall of the shell and outer walls of the tubes is many times larger than the total cross sectional area of the tubes inside the shell. To gain meaningful turbulent flow of the heat transfer medium in the shell side, exorbitantly large flow rates and/or extensive cross-sectional internal baffling is required within the tube shell, which in most cases still falls short of an uncompromised counterflow design which permits the heat transfer efficiency and small exit temperature differential capability that is typical for PHEX and spiral heat exchangers.
In consideration of the aforementioned circumstances, the present disclosure provides a liquid to liquid multi-pass countercurrent heat exchanger. It is understood that the use of a liquid to liquid multi-pass countercurrent heat exchanger is not limited to use for only liquid to liquid, but can be used for other mediums as well, such as gases.